1. Field of the Invention
The present invention relates to a circuit having the functions of continuously varying a bandwidth, and shifting the center frequency at an intermediate-frequency stage, in a double superheterodyne circuit demodulating a received signal after converting its frequency twice.
2. Description of the Prior Art
In conventional double superheterodyne circuits, a circuit having the function of varying the bandwidth in an intermediate-frequency stage mixes a received signal F.sub.1 with an oscillation frequency F.sub.OSC from a first local oscillator (variable) in a mixer M.sub.1 as shown in FIG. 1.
A frequency F.sub.2 obtained by the mixture and conversion passes through a filter F.sub.1 and is mixed with an oscillation frequency F.sub.W from an oscillator O.sub.2 in a mixer M.sub.2, and a frequency F.sub.3 thus obtained is supplied to a filter F.sub.2. By varying the oscillation frequency F.sub.W of the oscillator O.sub.2, the output frequency F.sub.3 of the mixer M.sub.2 varies according to the equation: EQU F.sub.3 =F.sub.W -F.sub.2,
between the upper and lower sides of the pass band of the filter F.sub.2. The frequency F.sub.3 passing through the filter F.sub.2 is mixed with the oscillation frequency F.sub.W from the oscillator O.sub.2 in a mixer M.sub.3 and thus returns to the original frequency F.sub.2.
The frequency F.sub.2 is mixed with a carrier frequency F.sub.C from an oscillator O.sub.3 at a detection stage D, and thereby an audio output is transmitted. That is, the composite band of the filters F.sub.1, and F.sub.2 is made to vary without varying the received frequency F.sub.1.
In this circuit the bandwidth does not vary much, since it has merely the constitution wherein the filter F.sub.1 is fixed and the oscillation frequency F.sub.W from the oscillator O.sub.2 is supplied to the mixers M.sub.2 and M.sub.3 provided on the input and output sides of the filter F.sub.2, respectively.
In addition, this circuit also has a fault in that a continuous variation of the bandwidth, and a shift of the central frequency, can not be conducted sufficiently.